1. Field of the Invention
The present invention relates to a technique for obtaining a closely spaced train of extremely high voltage short pulses which may be used for accelerating particles such as electrons, or for driving an undulator/wiggler.
2. Description of the Related Art
Electrons can be accelerated to high energies in a short distance by applying to a suitable accelerating structure a closely spaced train of extremely high voltage short pulses. The requirement of very short pulses is dictated by the need of sustaining ultrahigh electric fields in the accelerating structure, whether conducting or superconducting. Short pulses are usually produced in prior art techniques by switching a pulse forming structure on an electrical load. However, with very few exceptions, switching does not allow a high repetition rate of the high voltage pulses.
Many uses of fast repetitive pulses are described in the literature. For many of these applications, the repetition rate, i.e., the interval between two successive short pulses, is either too long, or the pulse train is composed of few short pulses. This is due to the fact that switching an energy storage device on a load stresses the switch at levels often close to destructive breakdown.
Accordingly, a need exists for a method for producing a closely spaced train of extremely high voltage short pulses which does not require switching as in prior art techniques.
The present invention is based upon the following rational and basic facts:
1) High fields can be held for short times in vacuum between metallic (normal) electrodes.
2) High fields can be held for short times in vacuum between high temperature superconducting electrodes.
3) High fields can be held for short times in vacuum between conventional superconducting electrodes.
4) An electric field changing in time has an associated magnetic field. In a plane wave, the ratio of the electric field intensity to magnetic field is simply the speed of light. A non-static high electric field is associated with a high magnetic field.
5) The critical magnetic field Hc (at which the transition superconducting-to-normal occurs) of a superconductor scales as the temperature difference between the transition temperature at zero field and the actual temperature. The highest transition temperature for normal superconductors is around 20 degrees Kelvin. The transition temperature of high temperature superconductors is around 100 degrees Kelvin. High temperature superconductors are the most promising candidate for efficient, ultrahigh field accelerators, because they can sustain high magnetic fields without reverting to normal conductors.
6) Superconducting electrode structures offer the highest efficiency of conversion from electromagnetic energy to accelerated beam energy. No other structure can offer the same efficiency. Conventional superconductors are used for acceleration, but with a maximum gradient of xe2x88x9230 MV/m (not yet limited by Hc, but close).
7) Pulse power technology produces extremely short pulses (less than one nanosecond and as short as 50-100 picoseconds). The repetition rate of these pulses does not exceed a few hundred/seconds. For many applications, this repetition rate is too low.
The present invention provides a method for generating a high frequency train of high voltage pulses by using a Fourier construction, rather than switching to produce the pulse train. Thus, the method of the invention involves generating the train of pulses by combining a plurality of harmonic amplitudes to construct the pulses, via a Fourier construction. Any arbitrary pulse shape can be reproduced simply by changing the amplitude of the harmonics.
The closely spaced train of extremely high voltage short pulses generated by the method of the present invention can be applied to conventional accelerating structures or, preferably, to superconducting structures for accelerating particles.
At lower power levels, the method of the present invention can be used to produce a train of ultrashort pulses that may have applications in many fields of electronics, for instance to drive solid state lasers at high peak power, or to drive Pockel cells, high power broad spectrum radar, etc.
At higher power levels, the train can be used to accelerate charged particles or to drive an undulator/wiggler, such as the undulator/wiggler disclosed in U.S. Pat. No. 4,972,420, reproduced here as FIGS. 8, 9A, 9B, 10A, 10B, and 11A, 11B.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.